1. Field of the Invention
This invention relates, generally, to wave-driven energy conversion devices that convert the abundant natural energy present in the oceans and other bodies of water into electrical or chemical energy.
2. Description of Prior Art
2a. Overview
2a1. Water Turbines
Water turbines have been used to extract useful energy from moving water, or water under pressure, for thousands of years. Many different types of water turbines have been invented and used in the past to extract energy from water under a variety of circumstances, e.g., Francis turbines, Pelton turbines, Kaplan turbines, etc.
The design of water turbines is a mature discipline. Most modern water turbines convert the kinetic and/or potential energies of water into rotary motion that can be used to create electricity. The efficiencies of these mature turbine technologies can be quite high, often exceeding 90%. And it is relatively easy to find an existing water turbine design that will optimally harvest the kinetic and/or potential energy available in almost any river or dam.
2a2. Wave Energy Devices
Attempts to extract energy from waves moving across the surface of an ocean are relatively new. Some devices of this type have been built, described in prior patents and proposed in the literature.
There are many good reasons to engage in such research and development efforts. Ocean waves represent a renewable energy source whose harvesting would not degrade the environment and ecology of the earth. Ocean waves also represent a very concentrated energy source, offering the potential for the harvesting of large amounts of energy using relatively small devices.
Most wave energy devices are constrained to use in relatively shallow ocean waters where they may be anchored in some manner to the ocean floor. When anchored to the ocean floor, a wave energy device may create and exploit a tension between the immovable ocean bed and the vertical oscillations of the waves. These types of devices are generally not capable of operating in the deeper parts of the ocean.
Among the many wave energy devices proposed or built, some are able to operate in the deeper parts of the ocean. Many of these devices utilize a buoy at the surface of the ocean, and a submerged component, which, in one way or another, exploits the relatively motionless waters which are found a short distance below the ocean's surface to facilitate the extraction of energy. Some of these devices utilize submerged turbines that are moved through the still waters by the action of waves above causing the turbines to rotate and generate power.
2a3. Energy in Waves
Waves traversing the surface of the ocean represent a repository of a large fraction of the total energy imparted to the earth by the sun. The sun heats the land and the oceans and much of this heat energy passes into the atmosphere. Differential heating of the atmosphere across the surface of the earth, in conjunction with the rotation of the earth, causes the atmosphere to move across the Earth's surface, sometimes at relatively high speeds.
When the atmosphere moves over the surface of the earth's lakes and oceans, it imparts some of its kinetic energy to the waters at the surfaces of those lakes and oceans, thereby creating waves on the surfaces of such bodies of water. The amplitudes of those waves increase as long as the wind blows parallel to the direction in which the waves are propagating. The uninterrupted distance over which the wind blows in a direction parallel to a wave's propagation, and over which it imparts increasing amounts of energy to that wave, is called the “fetch” of the wave.
Typical ocean waves range in height from three-tenths of a meter (0.3 m) to five meters (5.0 in). At higher (i.e. more polar) latitudes, ten-meter (10.0 m) waves are not uncommon.
2a4. The “Wave Base”
Water molecules and other particles contributing to the propagation of “deep-water” waves (i.e. those moving across waters with depths of about 50 feet or more) have circular orbits. (These orbits become elliptical as the water becomes shallower.) In deep-water waves, the radii of the orbits of the water molecules decrease exponentially with increasing depth. The radii become vanishingly small as the depth approaches one-half the wavelength of the waves. This special depth is called the “wave base.” A wave in the ocean does not move the water located below the wave base to any significant degree. The water below this depth and any objects floating in it are substantially stationary, even as waves move across the surface overhead.
It is possible to use the motion of waves at the surface of the ocean to move a submerged component up-and-down through the relatively still waters beneath the waves, e.g. beneath the wave base.
2a5. Extracting Power From the “Still” Water Beneath the Waves
The prior art includes a type of wave-energy device capable of extracting power by leveraging the motion of waves at the surface of the ocean against the still waters located beneath the waves UK Patent 45018/72 by J. Bichard, 1973. See FIG. 1. It includes a unidirectional or bi-directional propeller suspended from a buoy by a shaft or cable. As the buoy moves up and down in response to passing waves, the propeller is moved up and down through the relatively still waters below the surface. This up-and-down motion of the propeller through relatively still waters compels the propeller to spin. The propeller spins in a constant direction if the propeller is bi-directional but its direction of rotation reverses if the propeller is unidirectional. Such a device does not generate much power.
Even though the force driving the water back and forth through such a suspended turbine would be great, the speed of the water's movement though the turbine would be relatively slow. When driven by waves with a height of 4 meters and a period of 8 seconds, the maximum speed of a suspended turbine relative to the water around it would be about 1.6 meters per second. At this speed, it would be difficult to extract a significant amount of energy from the flowing water with a simple turbine because the amount of power that can be extracted from a flowing stream of water by a turbine is proportional to the cube of its speed. In other words,
Power available for extraction=0.5 A v3 
Power actually extracted=0.5 k A v3 
Where “A” is the cross-sectional area of the stream of water from which power is extracted (in this case, the cross-sectional area swept by the rotating blades of the turbine), “v” is the speed of the water moving through the turbine, and “k” is a constant that equals the efficiency of the turbine.
A reasonably sized device of the kind illustrated in FIG. 1 (e.g. a turbine diameter of 7 meters), with a turbine of reasonable efficiency (e.g. k=0.5), would only produce an average of about 16 kW when driven by waves with a height of 4 meters and a period of 8 seconds.
Since ocean waves rise and fall with a relatively slow speed (the maximum of which is generally only one or two meters per second), it is difficult to extract much energy from the water constrained to flow through a propeller at that same slow speed.
2b. Device Proposed by Heck
In “Wave Responsive Generator” (U.S. Pat. No. 4,447,740) Heck claimed a wave energy device that also suspended a turbine beneath a buoy like the one discussed above. However, Heck proposed surrounding his device's submerged turbine with a cylindrical housing designed to shield the turbine from damage from under-water debris. Heck further proposed (although he did not claim) a modification to his wave energy device in which frusto-conical sections would be added to the ends of the submerged cylindrical turbine housing. The effect of these frusto-conical sections would be to accelerate the speed of the water which entered the cylindrical turbine housing and which passed through, and powered, the enclosed turbine. Although Heck did not provide any specifics about the implementation of this modification, his illustrations provide some information.
According to the drawings, e.g. FIG. 1 of U.S. Pat. No. 4,447,740, which is duplicated in FIG. 2 of this patent, the frusto-conical extensions on the ends of the cylinder in Heck's device (Dt=1.16 Dh) would be rather small, and would be expected to increase the speed of the water passing through the associated turbine by 1.3× (a 30% increase) and to increase the power generated by the associated turbine by 1.8× (an 80% increase). (See Section 2c below.)
2c. Heck Device Water Speed and Power
Refer to FIG. 2.
Dt=diameter of the opening of frusto-conical section appended to the cylindrical turbine housing, i.e. the diameter of the mouth of the Heck device.
Dh=diameter of the cylindrical turbine housing, i.e., the diameter of the throat of the Heck device, and of the turbine enclosed therein.
At=cross-sectional area of the mouth of the Heck device.
Ah=cross-sectional area of the throat of the Heck device, and of the water flowing the enclosed turbine.
Vt=speed of the water entering the mouth of the Heck device.
Vh=speed of the water flowing through the Heck device, and through the enclosed turbine.
Pt=power available for extraction from the water entering the mouth of the Heck device.
Ph=power available for extraction from the water flowing through the throat of the Heck device.
k=efficiency with which the turbine of the Heck device extracts power from flowing water.
p=density of ocean water (i.e. 1025 kg/m3)
Pixel measurements of an image of FIG. 1 of Heck's U.S. Pat. No. 4,447,740, which shows the embodiment of his device that incorporates the frusto-conical sections, yield the following relative measurements:Dt=304 pixelsDh=262 pixelsDh=(262/304)Dt Dh=0.862 Dt Assume Dt=1.0 [a relative point of reference], therefore:Dh=0.862At=ΠDt2/4=Π/4=0.25 ΠAh=ΠDh2/4=Π0.8622/=0.186 ΠVh=(At/Ah)Vt=(0.25 Π/0.186Π)Vt=1.3 Vt 
The speed of the water passing through the turbine of Heck's wave energy device would be expected to increase by a factor of 1.3, or by 30%, as a result of the frusto-conical extensions appended to both ends of the cylindrical turbine housing, as illustrated in Heck's drawings.Pt=k 0.5 ρAtVt3=k0.5ρ(0.25Π)Vt3=0.125 k ρVt3 Ph=k 0.5 ρAh Vh3=k0.5ρ(0.186Π)(1.3 Vt3)=0.204 k ρΠVt3 Ph/Pt=(0.204 k ρΠVt3)/0.125 k ρΠVt3)=1.8
The 30% increase in the speed of the water passing through Heck's turbine would be expected to result in an 80% increase in the power available for extraction, and in the power which would ultimately be extracted.
In keeping with the example offered in section 2a5 (paragraphs 00019-00024), a reasonably sized Heck device of the kind illustrated in FIG. 2 (e.g. a venturi mouth with a diameter of 7 meters), with a turbine of reasonable efficiency (e.g. k=0.5) would produce an average of about 30 kW when driven by waves with a height of 4 meters and a period of 8 seconds (an 80% increase over the earlier power level of 16 kW—produced in the absence of any venturi device). However, the cost of constructing and maintaining a 7-meter diameter buoy and venturi device, including a 6-meter turbine, is unacceptably high for a device with an output of only 30 kW in rather energetic 4-meter oceans.
2d. Problems with the Heck device
The Heck device fails to generate enough power to justify its cost of construction, deployment and maintenance.
Another problem with the Heck device is its use of a single central pipe which encloses the turbine shaft and is also responsible for maintaining the attachment, and fixing the position (in a rigid manner), of the turbine and the cylindrical housing. The stress on this single central support would be much greater than necessary and introduce serious concerns regarding the ability of the device to survive the rigors of its time at sea.
Therefore a need exists for an apparatus which is capable of utilizing the energy of ocean waves to generate electrical energy which has the capability of accelerating the water flowing through a submerged component of the power generation system to the degree that sufficient power is generated to justify cost of construction, deployment and maintenance of the device and at the same time the device is sufficiently robust to withstand the stresses imparted thereto by the wave motion at the surface of the ocean.